The present invention relates to a process and apparatus for dewatering a suspension of solids in a carrier liquid by means of an electrically augmented vacuum filter (EAVF.RTM., a trademark of Dorr-Oliver, Inc.), including means for depositing solids from the bath of suspended solids onto an anodic structure and means for filtering filtrate from the bath of the suspension of solids at a cathodic structure. The cathodic structure comprising a cathode, catholyte, catholyte chamber, and filtrate chamber. The catholyte chamber having a membrane wall which is an ion exchange membrane, means for introducing and removing catholyte from this catholyte chamber, and means for removing the filtrate from the filtrate chamber. The membrane wall of the catholyte chamber may include either a non ion selective membrane or an anion exchange membrane according to the present invention. Moreover, the double chambered structure may be applied to the anode if it is desired that the anode be used to filter the filtrate from a suspension of solids.
The use an EAVF device for dewatering a suspension of solids in a carrier liquid is described in U.S. Pat. Nos. 4,168,222 and 4,207,158. These patents describe a means for dewatering a suspension of solids in an electric field which is controllably maintained between opposing electrodes, to cause the solids to migrate relative to the carrier liquid to form a layer or cake on one of the electrode structures in which the electrode elements are positioned within ion-pervious walls and immersed in a selected electrolyte, while allowing carrier liquid to be withdrawn under vacuum in the opposite direction through the liquid-pervious wall of a hollow, counter electrode structure, and wherein the layer or cake material may be detached from the first electrode structure during exposure from the suspension. The liquid-pervious wall of the electrode which is opposite the cake depositing electrode and which allows carrier liquid to be withdrawn therefrom under vacuum has the problem in that the filtrate drawn thru it is contaminated with the electrode reaction products. Also, the electrode having a liquid-pervious wall has a reduction in the flux of the filtration so that the amount of filtrate removed during a defined period of time is substantially reduced.
Typically, the filtrate withdrawal rate starts off very high, then drops to some equilibrium value after several days of operation. The filtrate rate will then be fairly constant, fluctuating based on variations in the feed composition, conductivity, and machine operating parameters. It is believed that the charged capillaries which are responsible for electro-osmotic pumping of filtrate became poisoned with hydroxide which was generated at the cathode. Thus, the high conductivity of hydroxide generated at the cathode surface diffuses into the capillary so that the filtrate rate declines. Because of the cathode assembly structure of earlier EAVF devices, where the cathode filter material is placed directly on a perforated electrode screen, it is felt that there are instances where back diffusion of hydroxide exceeds the forward flow of filtrate.
U.S. Pat. No. 4,312,729 (Wills) disclosed a cathodic structure comprising a catholyte chamber, perm-selective membrane and filtrate chamber. Wills attempted to prevent hydroxide generated from the cathodic reaction from passing into the feed slurry by providing a perm-selective membrane between the electrolyte chamber and the filtrate chamber. The cation selective membrane according to Wills attempts to prevent contamination of the feed suspension of filter cake with hydroxide and other ions which are deliterious to the suspension of the slurry. The perm-selective membranes of Wills are those membranes permeable to cations and substantially impermeable to anions, gases, water and other liquid.
The disadvantage of Wills is that the cation exchange membrane results in an extremely dialyzed filtrate since negative ions are transported out of the filtrate into the bath and are not replenished due to the impermeability of the membranes to anions resulting in an extremely high voltage drop across the filtrate chamber due to low conductivity and polarization layers. All the electrical current flow was forced by the cation exchange membrane to be carried across the boundary by positive ions. The high voltage drop is not very desirable due to its increased energy consumption.
The use of cation exchange membranes as shown in Wills and what was known by those skilled in the art in effect taught away from the use of an anion exchange membrane in a double chambered electrode for removing filtrate since it was expected that the filtrate would be enriched with hydroxide ions being transported through the membrane into the filtrate, which would poison the electroosmotic pumping of the electrode which Wills attempted to avoid. However, the present inventor observed that the membrane of the present invention permits the filtrate to become enriched in anions, but low in conductivity and near neutral pH which was unexpected.
Based upon extensive testing of different membrane structures the present invention overcomes the following disadvantages of the prior art: contamination of electrolyte, reduction in the flow rate of filtrate, dialyzing of the filtrate, and high voltage drop across the filtrate chamber due to low conductivity and polarization layers.
This and other advantages of the present invention will come clear as described below.